1. Field of the Invention
The present invention relates to a method for reducing the resin content in pulp produced by acidic decomposition during sulfite pulp cooking, and more particularly, to a method reducing the resin content in pulp produced by acidic decomposition during a sulfite pulp cooking step in an ozone bleaching process.
2. Description of Related Art
In order to meet the growing demand for paper pulp which has been produced entirely without bleaching using chlorine-containing chemicals, more and more paper pulp industries are becoming interested in the possibility of bleaching with ozone. Ozone is a very reactive chemical which has other effects in the process in addition to that which is primarily intended.
As is well known, the raw wood material which is used for producing paper pulp contains hemicellulose, lignin and extractive matter in addition to the sought-after cellulose. Of these substances, it is undesirable to have lignin and extractive matter in the final paper pulp product if the latter is to be of high quality. It can be said that the real purpose of cooking and delignification is to remove lignin and extractive matter from the chipping pieces so that the cellulose can be exposed. The release of extractive matter presents varying degrees of difficulty depending on the type of wood and on the pulp process. It is, for example, easier to prepare resin-free pulp from pine than from spruce, despite the fact that the extractive matter in pine wood is, in principle, more difficult to dissolve than that in spruce wood. This is due to the fact that, in its structure, pine wood has larger communication channels from the parenchymal cells, which contain the extractive matter, thereby facilitating transport from these cells.
The choice of process is also of great importance. Chemically prepared pulp can be subdivided into sulphate pulp, sulphite pulp and dissolving pulp. Sulphate pulp is produced by alkaline decomposition, which results in the extractive-matter content of the raw wood material not posing any direct problem, because the resin dissolves in the alkaline cooking liquid. When preparing sulfite pulp by acidic decomposition, the problem of separating off the resin is entirely different. The extractive matter is not dissolved so readily in the acidic cooking liquid, but instead tends to remain in the pulp. The resin then clings to the wire cloth when the paper is itself being manufactured, leading to holes being produced in the paper. Due to this, paper manufacturing has to be halted a couple of times a day in order to blow resin from the wire cloth. This leads to large losses in production.
When the pulp is used to manufacture food-stuff wrapping, for example so-called grease-proof paper, the resin also gives an undesirable taste to the paper/carton, which is also a very big disadvantage.
In principle, the preparation of dissolved pulp can be categorized separately, despite the fact that both sulphate pulp and sulfite pulp processes can be used. The cooking is carried out to very low yield, because the aim is that only pure cellulose, composed entirely of glucose, should remain in the pulp. Elimination of the resin is also of importance when preparing dissolved pulp. Methods which are used include, for example, storing of the chippings to permit so-called maturation of the resin, extraction by alkali with or without the addition of surfactants, and replacement of chlorine by chlorine dioxide in the pulp bleaching.
Swedish Patent 424884 (and 418628) of Mo and Domsjo AB, has proposed a process for decreasing the content of resin in cellulose pulps. According to this method, which is preferentially based on unbleached pulp, dewatering first takes place, where appropriate in several stages, to 15-35% by weight. Thereafter, alkali is added so that a pH of greater than 11 is obtained, and the pulp is processed mechanically in an apparatus having screws which rotate relative to each other. After this, the pulp is conveyed, using a pump, worm conveyor or belt conveyor, for example, to a tower or similar container, for further reaction with added chemicals. The dwell time for this stage of the reaction should be between 6 minutes and 5 hours. Thereafter, the pulp is washed in order to remove the resin which has been released.
While it is true that a substantial reduction in the resin content of the pulp can be achieved by using this method, the disadvantage is that the process is elaborate and requires a relatively extensive array of apparatus.
It can therefore be stated that there has hitherto been no simple method for removing resin from pulp, something which has been the curse of sulfite industries.